1. Field of the Invention
The present invention relates to a solid-state imaging device, a method for manufacturing the solid-state imaging device, and an electronic apparatus.
2. Description of the Related Art
Electronic apparatuses, e.g., digital video cameras and digital steel cameras, include solid-state imaging devices. For example, complementary metal oxide semiconductor (CMOS) type image sensors and charge coupled device (CCD) type image sensors are included as the solid-state imaging devices.
In the solid-state imaging device, a plurality of pixels are arrayed on a surface of a semiconductor substrate. In each pixel, a photoelectric conversion portion is disposed. The photoelectric conversion portion is, for example, a photodiode, and generates a signal charge by receiving light incident through an externally attached optical system with a light-receiving surface and effecting photoelectric conversion.
In the case where a color image is imaged in the solid-state imaging device, in general, the photoelectric conversion portion generates a signal charge by receiving light incident through a color filter with the light-receiving surface and effecting photoelectric conversion. For example, color filters of three primary colors, red, green, and blue, are disposed in a Bayer pattern on an imaging surface, and the light of each color passed through a color filter of each color is received by the photoelectric conversion portion in each pixel.
By the way, regarding the solid-state imaging device, miniaturization and, in addition, an increase in the number of pixel have been desired. In this case, the size of one pixel is reduced and, thereby, it becomes difficult that each pixel receives sufficient amount of light. Therefore, an improvement in image quality of the resulting image is not easy. Consequently, the solid-state imaging device is desired to have higher sensitivity.
In addition to the above description, in the case where high-speed imaging is executed to improve moving image characteristics and in the case where imaging in a dark place is desired, higher sensitivity is especially necessary because the amount of light incident on a pixel is reduced.
Regarding the CMOS type image sensor among solid-state imaging devices, a pixel is configured to include a pixel transistor besides the photoelectric conversion portion. The pixel transistor is configured to read a signal charge generated in the photoelectric conversion portion and output an electric signal to a signal line.
In general, regarding the solid-state imaging device, the photoelectric conversion portion receives light incident from the surface side of a semiconductor substrate, the surface being provided with circuits, wirings, and the like. In this case, the circuits, the wirings, and the like interfere or reflect the incident light and, thereby, it may be difficult to improve the sensitivity.
Consequently, a “backside-illumination type” has been proposed, in which the photoelectric conversion portion receives light incident from the backside opposite to the surface provided with the circuits, the wirings, and the like of the semiconductor substrate (refer to Japanese Unexamined Patent Application Publication No. 2008-182142, for example).
In addition to this, a “layered type” has been proposed, in which photoelectric conversion portions to selectively receive light of individual colors are not arranged in a direction along the imaging surface, but the photoelectric conversion portions for individual colors are arranged while being layered in a depth direction perpendicular to the imaging surface. Here, for example, the plurality of photoelectric conversion portions are formed from materials having band gaps different from each other and are layered in the depth direction, so that light of each color is isolated and detected and a signal is output on a color basis (refer to Japanese Unexamined Patent Application Publication No. 2006-245088, for example).
Furthermore, realization of an improvement of the sensitivity through signal amplification due to avalanche multiplication has been proposed (refer to IEEE Transactions Electron Devices Vol. 44 No. 10 October, 1997 (1997) and IEEE J. Solid-State Circuits, 40, 1847 (2005), for example).
Moreover, realization of an improvement of the sensitivity through the use of a “chalcopyrite based” compound semiconductor film, e.g., a CuInGaSe2 film, having a high optical absorption coefficient in a photoelectric conversion portion has been proposed (refer to Japanese Unexamined Patent Application Publication No. 2007-123720 and The Spring Meeting, 2008 by JSAP, Extended abstracts 29p-ZC-12 (2008), for example).